If from the command line UserA is able to create and remove a file on the external system:
touch /ftpdtm/MAexport/test.txt
rm /ftpdtm/MAexport/test.txt
The following may help:
Change the permission to the folder to 666 (or 766 if UserA really need the ability to execute something in the folder). 6 is read/write and no execute. Make sure that the group setting of the folder is one that the SAS MA Service Account is a part of. When SAS exports a file it's not writing it as the user. So assuming that UserA and the SAS MA Service Account are both part of the SAS group change the group of the folder to SAS if it's isn't already done.
If UserA wasn't able to create and delete a file on the external system with the touch and rm command then I would look at the details of the mount that connects the folder to another system.
On a Linux system I would execute the following:
cat /proc/mounts | grep MAexport
That should show you the user ID that the connection is based on. It should show you the user ID that the connection is based on. It will also show if the mount point is set up as read/writeable (rw). If the mount point is not set up as rw then that would need to be reconfigured and re-mounted. Whatever user ID is in place would have to be given the permissions on the external file system.
Hope that helps.